1. Field of the Invention
This invention relates to an information signal reproducing apparatus and more particularly to an apparatus for reproducing information signals from a recording medium on which pilot signals of different frequencies are recurrently superimposed in rotation on the information signals.
2. Description of the Related Art
In the field of high density magnetic recording which typically includes the video tape recorder (hereinafter referred to as VTR), it has recently come to be popularly practiced to have pilot signals of four different frequencies superimposed on information signals during recording and to carry out tracking control by using the pilot signals reproduced during reproduction of the information signals. The conventional signal processing operation which has been performed to obtain signals for tracking control is as briefly described below:
FIG. 1 of the accompanying drawings shows the tracks of recording magnetization formed on a magnetic record bearing medium by a VTR which is arranged to perform tracking control in the four-frequency method. FIG. 2 is a diagram showing the essential parts of a reproducing circuit arranged to obtain a tracking error signal. In FIG. 1, an arrow 2 indicates the travelling direction of a magnetic tape 1. Magnetic heads A and B which have a given azimuth angle are arranged to form recording tracks A1, B1, A2, B2 and so on. Another arrow 3 indicates the direction in which these heads perform a scanning action. In each of recording tracks 4, one of pilot signals of four different frequencies f1 to f4 is recorded together with the video signal for every field, that is, in each of the tracks one after another.
The sequence of rotation in which these pilot signals are recorded is as shown in FIG. 1. For example, the pilot signal of frequency f1 which is 102.5 KHz.apprxeq.6.5 fH is superimposed on the video signal in the track A1; the pilot signal of frequency f1 which is 118.9 KHz.apprxeq.7.5 fH in the track B1; the pilot signal of frequency f3 which is 165.2 KHz.apprxeq.10.5 fH in the track A2; and the pilot signal of frequency f4 which is 148.7 KHz.apprxeq.9.5 fH in the track B2 (fH: the frequency of horizontal synchronizing signal).
The frequency difference between the pilot signals recorded in adjacent recording tracks is either fH or 3 fH as shown in FIG. 1. When the head is scanning the recording tracks Ai (wherein "i" represents 1, 2, 3 and so on), the frequency difference of the pilot signal of the scanning track (the main scanning track) is always fH from that of the track on the right-hand side as viewed on FIG. 1 and is always 3 fH from that of the left-hand track. Further, when the head is scanning the recording tracks Bi ("i": 1, 2, 3, . . . ), the frequency difference of the pilot signal of the scanning track is always 3 fH from that of the right-hand track and is always fH from that of the left-hand track.
Since all the pilot signals (of frequencies) f1 to f4 have relatively low frequency values, the head can reproduce, as cross-talk, the pilot signals from tracks located adjacent to a track mainly scanned or traced. For example, when the head is mainly scanning the track A2, the pilot signal thus detected (or reproduced) is a composite signal consisting of the frequency components f4, f2 and f3 representing these pilot signals. In case that the head is in a so-called on-track state with the center of the tracing locus of the head precisely coinciding with the center line of the main scanning track, the pilot signals f2 and f4 of the two adjacent tracks are at the same level. The reproduced level of the pilot signal f4 becomes higher than that of the pilot signal f2 when the head slightly deviates from the center line of the track A2 toward the track B2. The reproduced level of the pilot signal f2 becomes higher than that of the pilot signal f4 when the head slightly deviates toward the track B1.
Therefore, the direction and extent of deviation of the head from the main scanning track are obtainable by separately taking out difference signals fH and 3 fH representing differences between the pilot signal of the main scanning track and the pilot signals of the two adjacent tracks respectively and by comparing the levels of the difference signals thus obtained with each other.
FIG. 2 shows in a block diagram a circuit of the above-stated four-frequency method. A terminal 5 is arranged to receive a reproduced signal in which the pilot signals are superimposed on the video signal. A low-pass filter (hereinafter referred to as LPF) 6 is arranged to separate the pilot signal component from the reproduced signal. A local pilot signal generating circuit 7 is arranged to generate local pilot signals. A multiplier 8 is arranged to perform a multiplying operation on the pilot signal component and the local pilot signal generated by the generating circuit 7. The local pilot signal is of the same frequency as that of the pilot signal recorded on the main scanning track. As mentioned with reference to FIG. 1 in the foregoing, assuming that the main scanning track is the track A2, the output of the LPF 6 includes frequency components f2, f4 and f3. Meanwhile, the local pilot signal is of the frequency f3. Therefore, the multiplier 8 produces a signal of a frequency representing the sum of and a difference between the frequency f3 and the frequencies f2, f4 and f3. A band-pass filter (hereinafter referred to as BPF) 9 is arranged to take out only a signal of fH from the sum and difference signal while another BPF 10 is arranged to take out only a signal of 3 fH from the sum and differrence signal. The outputs of the BPF's 9 and 10 are detected and rectified by detection circuits 11 and 12 respectively. Following that, the signal components fH and 3 fH thus obtained are supplied to a level comparison circuit 13. The circuit 13 then produces a signal corresponding to a level difference between these signal components. More specifically, in case where the reproduced level of the signal of the frequency fH is higher than that of the signal of the frequency 3fH, the output of the circuit 13 is of a positive potential corresponding to the level difference. In the event of a case opposite to that, the output of the circuit 13 is of a negative potential. The signal produced from the level comparison circuit 13 can be used as a tracking error signal as it contains information on the extent and direction of the deviation of head from the track.
As apparent from the foregoing description given with reference to FIG. 1, the deviating direction of the head from the tracks Ai is opposite to the deviating direction from the tracks Bi. Therefore, a switching circuit 16 is arranged to selectively produce the output of the level comparison circuit 13 either directly or through an inverting amplifier 14 according to a head change-over signal 15.
While tracking control can be adequately accomplished for reproduction of a video signal under a normal condition, let us consider now a tracking signal processing operation under a transient condition. With the main scanning track assumed to be the track A2 (see FIG. 1) in the beginning of reproduction and the local pilot signal component used for the multiplication process assumed to be of the frequency f1, the detection circuits 11 and 12 respectively produces the frequency components fH and 3 fH in the manner similar to the tracking control under the normal condition. In this instance, however, the tracking control is performed in such a manner as to cause the reproducing head to further deviate from the track A2 because the polarity obtained in this instance is opposite to the polarity obtained under the normal condition. Then, the pilot signal recorded in the main scanning track and the local pilot signal eventually come to coincide with each other.
However, with the levels of the components fH and 3 fH becoming about the same while the reproducing head is justly tracing the track A2, when the reproducing head is shifted either in the direction of arrow 2 or in the direction opposite thereto, a considerably long period of time is also required before the pilot signal of the main scanning track and the local pilot signal come to coincide with each other. Therefore, the conventional circuit arrangement has presented a problem that, during the above-stated long period of time, the quality of the reproduced picture tends to degrade. Further, in accordance with the conventional method described, the level difference between the pilot signals which are for example f2 and f4 and recorded in the two adjacent tracks B1 and B2 with the track A2 assumed to be mainly scanned or traced, is arranged to be detected in obtaining the tracking error signal. However, since the pilot signals f2 and f4 are reproduced as cross-talk and thus do not have any dynamic range, they are poor in the S/N ratio. Thus, it has been impossible to carry out tracking control with a sufficient degree of accuracy.